The present invention relates to a wire cut electrical discharge machine, and in particular, to an improved wire cut electrical discharge machine having a higher accuracy.
FIG. 1 is a schematic diagram showing an arrangement of a prior art wire cut electrical discharge machine.
In the Figure, reference numeral 1 designates a wire-form electrode, 2 a workpiece, 3 an X-slider for moving the workpiece 2 in right-left directions as viewed in the FIG. 4 a Y-slider for moving the workpiece 2 in front-back directions as viewed in the FIG. 5 a servomotor for driving the X-slider, 6 a servomotor for driving the Y-slider 4, 7 a servo-amplifier for supplying a current to the servomotor 5, 8 a servo-amplifier for supplying a current to the servomotor 6, 9 a power supply for machining for applying a pulse-shaped voltage between the wire-form electrode 1 and the workpiece 2, 10 a detector for detecting an average machining voltage applied between the wire-form electrode 1 and the workpiece 2, and 11 a control unit for controlling the servo-amplifiers 7 and 8 in accordance with a signal from the detector 10 and a predetermined machining program.
Next, the operation will be described. The machining is performed by travelling the wire-form electrode 1 at a predetermined speed, by applying the pulse-shaped voltage between the wire-form electrode 1 and the workpiece 2 from the power supply for machining 9, and producing a discharge between the wire-form electrode 1 and the workpiece 2. In this case, movement command signals are transmitted to the servo-amplifiers 7 and 8 in accordance with a programmed locus provided to the control unit 11 beforehand, and the servomotors 5 and 6 respectively control the X-slider 3 and the Y-slider 4 thereby to machine the workpiece 2 to a desired shape. Generally, since the machining condition changes frequently, the control unit 11, in accordance with an average voltage between the poles (anode and cathode) detected by the detector 10, drives the X-slider 3 and Y-slider 4 at optimum feed speeds so that the machining clearance between the wire-form electrode 1 and the workpiece 2 becomes constant. Normally, it is possible to obtain a satisfactory geometry accuracy and face roughness by performing face finishing several times after roughing. In this respect, the geometry accuracy after the finishing is determined by the electrode side gap (clearance between the side of the electrode and the workpiece). For this reason, in order to achieve the shape machining with a high accuracy, it is necessary to maintain the electrode side gap at a constant value.
FIG. 2 is an enlarged view of the wire-form electrode 1 and the workpiece 2 during the finishing. In a conventional, usual manner of control of the average voltage at a constant value, the machining speed U decreases when the amount of removal L is increased. As a result of this, the machining integral effect at a side portion of the wire (a portion D in FIG. 2) increases and the electrode side gap g.sub.s increases. In other words, even when the machining electrical conditions and the average servo-voltage are maintained unchanged, if the amount of removal L is changed, the electrode side gap g.sub.s will not be held constant resulting in the degradation of the geometry accuracy after finishing. FIG. 3 shows a relationship between the amount of removal L and the electrode side gap g.sub.s when the machining electrical conditions and the average servo-voltage are not changed. It will be seen from the Figure that the electrode side gap g.sub.s changes to a great extent depending on a change in the amount of removal L. In actual shape machining, a change in the amount of removal L becomes maximum at a corner portion of the workpiece. FIG. 4 is an enlarged view of the wire-form electrode 1 and the workpiece 2 during incorner machining, where R is a wire radius, r is a face of the previous machining, r` is a radius of a wire locus, L.sub.0 .about.L.sub.5 indicate the amount of removal at each of wire center positions O.sub.0 .about.O.sub.5. It will be seen from the Figure that the amounts of removal L.sub.2 .about.L.sub.4 at the corner portion change to large values as compared with the amounts of removal L.sub.0 and L.sub.5 during straight machining. FIG. 5 illustrates a change in the amount of removal L at the incorner portion of the workpiece. In the Figure, the amount of removal L begins to increase at a certain distance H1 before the beginning of the corner portion, and it is maintained at a constant value for a while. Then the amount of removal L begins to decrease at a certain distance H3 before the end of the corner portion, and it reaches again the amount of removal at the straight machining portion of the workpiece. As described in the foregoing, in particular at the incorner portion, since the enlargement of the electrode side gap g.sub.s occurs due to the increase in the amount of removal L, the geometry of the workpiece after machining is degraded to a great extent as shown in FIG. 6 wherein d represents the amount of overcut. Moreover, at the outcorner portion of the workpiece, since the decrease of the electrode side gap g.sub.s occurs due to the decrease in the amount of removal L, the geometry of the workpiece after machining is likewise degraded.
Because the prior art wire cut electrical discharge machine is arranged as described above, problems are involved in that the wire electrode side gap is changed due to a change in the amount of removal occurring in particular at the corner portion and the like of the workpiece, and consequently the accuracy of the geometry after machining is degraded to a great extent.